After upgrading from a MATE desktop system running Fedora 41 to Fedora 42 using the system upgrade option, the default login screen has a black background and the Fedora 42 desktop background is not used.

The `/etc/lightdm/slick-greeter.conf` file references a default background of `/usr/share/backgrounds/default.png`, but this file does not exist. Instead, the `/usr/share/backgrounds` directory contains symlinks such as `default-dark.jxl`, `default.jxl`, and `default.xml`.

If I create a symlink `/usr/share/backgrounds/default.png` that points to `./f42/default/f42-01-day.jxl` (yes, this is a mismatched filetype), the background is shown. Or, I can provide a symlink that points to an older Fedora background that *is* in *png* format, which also works.

Can slick-greeter be adapted to support the JXL files that are included with Fedora 42?

Reproducible: Always

Steps to Reproduce:
1. Install MATE desktop with lightdm and slick-greeter-mate
2. Logout or reboot
3. Observe black background for the login page
Actual Results:
There is no background image on the login page

Expected Results:
Fedora 42 default background would be used for the login page